In recent years, keyless entry for facilitating unlocking and locking of a car has been adopted in many cars. This technique performs unlocking and locking of a door using communication between a key of an automobile and the automobile. Further, in recent years, a smart entry system that makes it possible to perform, with a smart key, unlocking and locking of a door lock and start an engine without touching a key has been also adopted.
However, a lot of incidents occur in which an attacker intrudes into communication between a key and an automobile and steals the automobile. As measures against the attack (so-called relay attack), a measure for measuring the distance between the key and the automobile and, when determining that the distance is equal to or larger than a predetermined distance, prohibiting control of the automobile by communication is examined.
As a distance measuring technique, many techniques exist, such as a two-frequency CW (continuous wave) scheme, an FM (frequency modulated) CW scheme, a Doppler scheme, and a phase detection scheme. In general, in distance measurement, a distance from a measuring device to a target object is calculated by providing a transmitter and a receiver in the same housing of the measuring device, hitting a radio wave emitted from the transmitter against the target object, and detecting a reflected wave of the radio wave with the receiver.
However, when taking into account a relatively small reflection coefficient of the target object, limitation on output power due to the Radio Law, and the like, in the distance measuring technique for measuring a distance using the reflected wave, a measurable distance is relatively small and the technique is insufficient to be used in the measures against the relay attack.